1. Field of the Invention
The invention relates to a process for the optical detection of objects moved at a conveying speed.
2. Description of the Related Art
The detection and subsequent sorting of bulk materials with color cameras is an established method. A common embodiment variant is described, for example, in Patent AT 410847. This technology is used in particular in the field of recycling. In doing so, a transmitted light detection and a sorting are predominant in which glass and transparent synthetic products are processed by radiating light through them, collecting the light passing through by means of the color camera and analyzing it. In this technology, however, labels or the like on the transparent glass and plastic parts constitute considerable obstacles so that costly mechanical methods, see, e.g., AT 503036 B1, are even employed for their removal in a pre-stage of the process. However, for improving the yield, it is absolutely desirable that transparent objects are identified although non-transparent foreign matters, such as labels, stick to them.
Furthermore, fluorescent tube systems or LED-based lighting systems (disclosed, e.g., in utility model AT 004889U1) are also used nowadays. The advances in optical technology provide color cameras with higher and higher line scanning rates which, however, in turn require shorter and shorter illumination times and thus higher luminances for illuminating the objects to be detected. The high luminances required for systems with line scanning rates of 4-20 kHz, which are already prior art or will be available in the near future, can no longer be achieved with fluorescent tubes or thermal illuminants. Therefore, it has already been suggested that systems be equipped with light emitting diodes (LEDs) as illuminants in order to increase the luminance. Beam focussing optical systems in combination with a generation of white light by means of blue LEDs with yellow fluorescent dyes (so-called white light LEDs) have become established as a standard in this field. However, due to the inefficient color conversion, this type of illumination requires an active cooling, which is accomplished, for example, by water coolings in a costly manner.
Color cameras with color filters on the optically sensitive elements (pixels) or 3-chip cameras are today used as standard technology in connection with white light illumination, wherein one chip each is sensitive to one of the three primary colors red, green, blue. Furthermore, the demand made on the resolution and hence the number of pixels is getting larger and larger. Today, more than 1000 color triples are already standard, and developments are aimed at raising the limit by one order of magnitude. This requires a further reduction in pixel size, which, in turn, further increases the demands made on the luminance of the lighting equipments, and also a decrease in the quality of the color filters, which are questionable, anyway. In 3-chip cameras, the required optical splitting of the optical paths for separating the color components and the adjustment of the sensors relative to each other are to be assessed as critical.
Thus, the need for an improved process for the optical detection of moving objects still exists which provides a high quality of identification at high conveying speeds of the objects to be detected, but at the same time is able to get by with lighting equipments of comparatively low luminances.